1. Field of the Invention
This invention relates to an electrophotographic printer for printing an image on a recording medium through an electrophotographic process and, more particularly, to an electrophotographic printer capable of changing printing speed thereof.
2. Description of Related Art
A electrophotographic printer generally forms an electrostatic latent image on a surface of an electrically charged photosensitive drum by light beam from an emitting source, develops the image by using toner clinging to the electrostatic latent image, and transfers the image by the toner onto a recording medium, such as a paper.
Referring to FIG. 1, such a conventional electrophotographic printer is shown. In this printer, a photosensitive drum 11 rotates in an arrow direction by a drive motor not shown, and keeps its rotation in a constant speed. Using a constant voltage, a charging roller 12 charges the surface of the photosensitive drum 11 uniformly. A laser scanner 13 has its scanning period corresponding to the rotation speed of the photosensitive drum 11, and exposes the surface of the drum 11, sequentially, by scanning motion thereof. By this exposing process, an image, or data to be printed, is formed as an electrostatic latent image on the drum 11. A developer 14 makes toner selectively adhere to the electrostatic latent image on the drum 11 to form an image by the toner. A transfer roller 15 is connected to a constant current source so that a high voltage from the constant current source is supplied to the surface of the roller 15. The transfer roller 15 attracts the developed image onto a recording medium, such as a paper 16, by electrostatic force, and then, transfers the image to the paper 16. A cleaning apparatus 17 eliminates toner remaining on the drum 11 after the image is transferred, and collects the remaining toner. Fixing rollers 18 fuse the toner by heat on the passing paper 16 to fix the toner thereon. The temperature of the fixing rollers 18 is monitored by a thermistor to keep it. The feeding speed of the paper 16 is identified with the speed of the surface of the photosensitive drum 11 and with the speed of the surface of the fixing rollers 18.
In such a conventional electrophotographic printer, it is difficult to change the speed of printing since the rotation speed of the photosensitive drum 11 is set to a certain constant speed. That is, the laser scanner 13 uses a polygon mirror by which the laser beam is deflected to form an electrostatic latent image. To change the speed of a motor for rotating the polygon mirror, the drive mechanism of the polygon mirror requires a response time of several seconds or above. Such an electrophotographic printer, therefore, needs a time for inhibiting printing for several seconds or above, in the case when the photosensitive drum 11 changes its rotation speed without changing its resolution, or its scanning density. In addition, it is difficult in such an electrophotographic printer to comply with operations of respective processes, such as transferring, fixing and the like, with operation of the photosensitive drum 11 when the photosensitive drum 11 changes its rotation speed, and printing thereby tends to be degraded.